The present invention relates to a clutch-separable integral-type planetary gear transmission including three planetary gear trains with two elements in each gear train being coupled together.
Planetary gear transmissions are widely used as automatic transmissions for automobiles. Many conventional planetary gear transmissions comprise an integral combination of two planetary gear trains, as with the Ravigneaux gear train, the Simpson gear train, or the like, and generally have up to four forward gear positions. For better running characteristics of automobiles, there have been proposed and used transmissions having five or more forward gear positions.
Such multiple-gear-position transmissions are disclosed in Japanese Laid-Open Patent Publication No. 63(1988)-318349, Japanese Laid-Open Utility Model Publication No. 61(1986)-103654, for example. The disclosed transmissions comprise two planetary gear trains, three clutches and a brake combined with each of the planetary gear trains, and have six forward gear positions and a single reverse gear position. Since there are only two planetary gear trains, the above disclosed transmissions and the conventional transmissions may employ common components. However, the disclosed transmissions must be controlled according to a complex control process because some gearshifts require simultaneous disengagement of two engaging components (a clutch and a brake) and engagement of other two engaging components.
For example, when the disclosed transmissions are shifted from the second gear position to the third gear position or vice versa, it is necessary to disengage one clutch and one brake and also to engage another clutch and another brake.
Japanese Laid-Open Patent Publications Nos. 59(1984)-222644 and 1(1989)-320362 show transmissions with three planetary gear trains. In each of the disclosed transmissions, two components of each of the planetary gear trains are mechanically coupled to components of the other planetary gear trains, and four clutches and three brakes are coupled to the planetary gear trains. The transmissions have five forward gear positions and one reverse gear position which can be established by controlling operation of these engaging components, i.e., the clutches and the brakes. All successive gearshifts between the five forward gear positions can be achieved by engaging one engaging component (a clutch or a brake) and engaging another engaging component. Therefore, these transmissions can be controlled relatively simply.
However, the transmissions with three planetary gear trains are disadvantageous in that either one of elements (i.e., a sun gear, a carrier, and a ring gear) of the planetary gear trains overspeeds, i.e., rotates at a considerably higher speed than the rotational speed of the engine (i.e., the input rotational speed), in some speed ranges (e.g., a LOW (1st) range, a 5th range, and a REV range). While these transmissions can be used in combination with low-speed engines such as diesel engines, difficulty is experienced in combining them with high-speed engines.
For increasing gear positions from the four gear positions to five gear positions, the range of speed reduction ratios, i.e., from the first gear position (LOW) to the fifth gear position, is required to be greater than the range of speed reduction ratios from the first gear position to the fourth gear position for improved running characteristics of automobiles. If the range of speed reduction ratios is widened, however, the above overspeed problem tends to become worse.